Signal Propagation

sender

Propagation: How the Signal are spreading from the receiver to sender.

Transmitted to the Receiver in the spherical shape.

• When the signal transmit from sender to receiver , the character of signal may not be changed in the wired network.

•But wired signal there is a change in character, like packet loss, external factors .etc,.

Signal Propagation Ranges

distance

sender

transmission

detection

interference

Transmission range communication possible low error rate

Detection range detection of the signal

possible no communication

possible

Interference range signal may not be

detected signal adds to the

background noise

Path Loss of Radio Signals

When Propagation in free space always like light (straight line).

The signal passes sender to receiver in a straight line is called Line-of-sight (LOS)

If there is a vacuum between sender receiver , it’s a free space loss.

Received power by the receiver is P. Distance between sender and receiver is d.

The received power by the receiver

Pr is proportional to 1/d2.

Pr α 1/d2.

The sender emits signals with certain energy.

If there is a vacuum space, the sends at the speed of light with spherical shape.

If there is no obstacle continuously grows the surface of sphere (s) and it moves to the

particular distance d. According to the equation

s=4πd2

Path Loss of Radio Signals

The additional parameters are important .

The receiver power depends on the wavelength and gain of receiver and transmitter.

The external factors are affects the signals . The path loss not occures in short distance.

The atmosphere influences in long distance travel signal.

Heavy Rain - Mobile Phone

Depending upon the frequency , it penetrate objects

A short wave length with lower frequencies

penetrate easily

A long wave length with higher frequencies can

be blocked by external factors

FUNDAMENTAL PROPAGATION BEHAVIORS

Ground Wave

• < 2 MHz . LF and MF • Low frequency follow the earth surface • Used in Submarine Communication and AM Radio

Sky Wave

• 2 to 20 MHz • Waves bounce from the ionosphere to earth. The short waves are reflected• Used in International broadcasts

SIGNAL PROPAGATION

Shadowing Making the signal weak

Reflection at large obstacles Refraction

Depending on the density of a medium Changing in Direction

Scattering at small obstacles The signal meets one point and scatter as many

Diffraction at edges

Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction. i.e radio signals reaching the receiving antenna by two or more paths

The signals travelling along different path, different length and different time. This effects is delay spread

MULTIPATH PROPAGATION

A Real Example of Multiplexing

How ???

MULTIPLEXING

Transmission of multiple data communication sessions over a common wire or medium

Minimum of interference and a maximum of medium utilization.

s2

s3

s1

Space Division Multiplexing

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channels ki•The channels k1 to k3 can be mapped onto the three ‘spaces’ s1 to s3•Separate the channels and prevent the interference ranges from overlapping

k4 to k6 is allocated for some others.

Analog Telephone

Frequency Division Multiplexing

• The total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands

• Each of which is used to carry a separate signal

Frequency Division Multiplex

Separation of the whole spectrum into smaller frequency bands

Advantages: no dynamic coordination

necessary works also for analog signals

Disadvantages: waste of bandwidth

if the traffic is distributed unevenly

inflexible guard spaces

E.g FM Radio

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TIME DIVISION MULTIPLEXING

• The time domain is divided into several recurrent time slots of fixed length• A sample byte sub-channel 1 is transmitted during time slot 1, sub- channel 2 during time slot 2, etc

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Time Division Multiplex

A channel gets the whole spectrum for a certain amount of time

Advantages: only one carrier in the

medium at any time throughput high even

for many users

Mobile Phone for a Time

f

Time and frequency multiplex

Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM

Advantages: better protection against

tapping protection against frequency

selective interference higher data rates compared to

code multiplex but: precise coordination

required

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Code Division Multiplexing (CDM)Also known as code division multiple access (CDMA)

An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes

Used for mobile communications.

Each mobile device is assigned a unique 64-bit code (chip spreading code)

To send a binary 1, mobile device transmits the unique code

To send a binary 0, mobile device transmits the inverse of code

Code Division Multiplex

Each channel has a unique code

All channels use the same spectrum at the same time

Advantages: bandwidth efficient no coordination and synchronization

necessary good protection against interference and

tapping Disadvantages:

lower user data rates more complex signal regeneration

Implemented using spread spectrum technology

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